The present invention relates to a method for operating a machine tool and to a machine tool operated in accordance with this method. More particularly, the invention relates to a method of controlling relative movements of a tool held in the machine and a workpiece after an unexpected or otherwise unplanned interruption of the machining operation occurred.
Modern machine tools for metal cutting machining of workpieces are capable of carrying out a large number of complex machining operations in an automated manner. These machining operations can include, for example, milling, turning, boring, grinding and others. A plurality of machining tools and types of machining tools can be clamped into a tool carrier. The tool carrier typically comprises an operating spindle, which brings the clamped tool into a rotating movement (inter alia during boring and milling). In many cases, the operating spindle can additionally be moved in a number of spatial directions, so that it is able to act at various spatial points on the workpiece to be machined. However, the tool carrier can also be rigid and interact with an appropriately moved workpiece holder, which is in particular the case in turning. Furthermore, the workpiece can also be moveably held. All that matters in the cotext of the present invention is the relative movement between the tool and the workpiece. These relative movements take place along what are known as NC axes.
Generally, the machining of a workpiece is controlled by what is known as a NC control (numerical control). A (first) control program, which is known as the NC program, is loaded into the NC controller by the operator of the machine tool. The NC program contains a large number of control sets to be carried out successively, wherein the relative movements between tool and workpiece are controlled by the control sets for each machine step.
Responsibility of the NC controller in its original sense is restricted to the movements of the NC axes. Accordingly, various auxiliary units belonging to the machine tool are controlled by a second controller, which is typically known as a PLC (programmable logic controller). The auxiliary units include, in particular, a tool magazine, in which the necessary tools (mills, drills, turning tools etc.) are stored, a tool changer for inserting and removing tools from the magazine and the tool carrier, a pivotable working table, on which the workpieces can be clamped, or units for the supply of coolant, compressed air and others. It goes without saying that the control processes of the NC controller and of the PLC have to be coordinated well, and this is generally done by means of data interchange via suitable interfaces. The NC controller and the PLC are sometimes combined in one structural unit in today's machines, so that there is physically only one control unit but which combines both functionalities in itself.
Modern machine tools of the type described before are capable of producing workpieces at high machine speeds and with constant quality. The faster the machine tool operates, the more items can be produced. However, the production sequence, which is already very efficiently configured nowadays, is severely disrupted if, during the operation of the machine tool, unexpected/unplanned interruptions occur, such as a power failure, an emergency stop triggered automatically or by hand, erroneous operation in the form of an unintended manual intervention or an overload situation which, for example, is caused by a tool breakage. In such events, the programmed movement sequence is interrupted more or less abruptly. In order to resume the production sequence, the machine tool must first of all be brought by hand into a starting state again, from which a restart and, in particular, re-entry into the NC program can be carried out. Depending on the cause of the unplanned stop and depending on the current operating situation in which the interruption occurred, however, it is currently difficult to place a machine tool in the suitable starting state. The stoppage times associated with this are detrimental to the production sequence. In addition, there is the risk of a “crash” because of an erroneous operation in this situation.
WO 00/66320 suggest to monitor the power supply of a machine tool in order, in the event of a power failure being detected, to activate a mechanical energy store, which maintains the power supply for a certain time interval. Using the mechanically produced “residual energy”, the operating spindle is moved into a park position. In this way, restarting the machine tool after a power failure is already made substantially easier.
However, this approach is restricted to situations in which the electric power supply fails unexpectedly. Furthermore, there are other approaches according to which a controlled stop is carried out within the first seconds or fractions of a second after an emergency stop command has been triggered. However, in spite of such individual measures, there remains a whole series of scenarios in which a restart of the machine tool is complicated and lengthy, which can lead to long down times of the machine.